I mean the classic is actors saying "break a leg" - which actors actually take very seriously.

What area of the world do you live in? In my neck of the woods wishing a "boring shift" or a "quiet shift" is considered a curse and will bring about all the crazy cases.

We typically wish each other a "safe shift" instead of a "quiet shift" here.

One that my DM and I came up with was a common farewell in Orc and Hobgoblin culture.
"Fight well!" "Die better!"

Is the Scottish play superstition only for theaters which are currently rehearsing/performing that play, or is it an all-encompassing sort of He-Who-Shall-Not-Be-Named situation, where the proper title is never utilized if it can be avoided?

The latter; and it's taken seriously by many, less seriously by many others. For some, they will never utter the name; others simply don't say it in a theatre. My wife is well into the second camp, and for example, while explaining it to kids in kids camp at the theatre, was quite willing to say the name.

For me, my main basically hard-and-fast rule is not saying it to actors or techs during rehearsals or productions, but that is mainly because for the ones who DO believe in it, it might make them nervous enough to actually do something that they would attribute to the curse. Placebos have real effects. :)

"Hope there are no notes." Audio Post-production before client playbacks.

Fascinating. The first time I ran across the superstition was in a high school English class, in which the teacher would wheel out the TV and her Slings and Arrows dvds whenever she didn't want to do the lesson. I don't recall much (other than it being quite funny), but the "curse" of the play (and it's legitimacy) came up frequently over the course of one of the seasons.

Working in a restaurant the word quiet will not leave my mouth as long as I'm working, and the moment someone else says it I know we're about to have a horrible shift

Blue skies and safe landings.

I would never say quiet, hence boring. Also reminds people that a shift that drags a little is better than an interesting one.

Ah, makes sense! Perhaps a dumb question, but I was curious for an answer from someone with experience vs assuming, so thank you :)

I have been in IT too long. As soon as 500 miles appeared I said "very short timeout"

I'm in a giant organisation. All our releases are on Fridays and Saturdays to give Sunday for user acceptance test. Our big releases are Christmas and Easter for the extra time they offer for UAT. Last time I was on duty for UAT my slot was 02:00 to 02:30 boxing day, and the product hadn't been deployed correctly